1 / 16

Thermochemistry Coffee cup calorimetry Energy content of fuels

Thermochemistry Coffee cup calorimetry Energy content of fuels. In addition to this presentation, before coming to lab or attempting the prelab quiz you must also: Watch the prelab videos (#4 and 5) for this experiment Read the introduction to the lab in the coursepack. What’s the point?.

verne
Download Presentation

Thermochemistry Coffee cup calorimetry Energy content of fuels

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Thermochemistry Coffee cup calorimetryEnergy content of fuels • In addition to this presentation, before coming to lab or attempting the prelab quiz you must also: • Watch the prelab videos (#4 and 5) for this experiment • Read the introduction to the lab in the coursepack

  2. What’s the point? • See practical applications of thermochemistry • hot packs • fuels • Perform coffee cup calorimetry • Practice calorimetry calculations (Chapter 5 Brown, LeMay, Bursten)

  3. Thermochemistry study of energy flow during chemical reactions when heat is produced from a reaction, the reaction is called exothermic when heat is required for a reaction to proceed, the reaction is endothermic Our experiments focus on the production of energy in the form of heat as a result of dissolving a salt in water combusting a fuel Background

  4. Heat (q) • An energy transfer between system and surroundings due to temperature difference • Heat flows from “hot” to “cold” • but heat flow cannot be measured directly • instead, the effects of heat flow are measured • relate temperature change and heat flow via specific heat capacity

  5. Mathematically, q = m s T q = heat (joules J) m = mass (grams) s = specific heat capacity (J g-1oC-1) the heat required to raise 1 gram 1 oC T = change in temperature (oC) DT = Tfinal – Tinitial

  6. Coffee Cup Calorimetry • We will measure the temperature change that occurs upon dissolving CaCl2 in water: CaCl2(s)  Ca2+(aq) + 2Cl-(aq) • Since this is at constant pressure, we will be measuring the enthalpy (DH = qP) • Since this is a dissolution reaction, we call this enthalpy the “enthalpy of solution”

  7. Well insulated Poor stirring Poorly insulated Poor stirring Well insulated Good stirring Estimating the Maximum DT • To make an accurate measure of the temperature change, you must have a well insulated container or heat will be lost to the air during the experiment • You also want to have efficient stirring so that the salt dissolves quickly. This heats the solution quickly, which also helps minimize heat loss

  8. Linear portion of cooling curve Estimated maximum temperature for perfectly insulated and stirred calorimeter Linear portion of heating curve Estimating the Maximum DT • We will “interpolate” the maximum temperature by drawing lines through the heating and cooling portion of the curves as shown below (Tmax ~ 84 oC).

  9. Sample Calculation • Calculating DH requires that you know • the temperature change of solution • the mass of solution • the specific heat capacity of solution • the moles of salt reacted • When 7.85 g of NaCl is dissolved in 50.0 g of water, the solution temperature drops by 2.07 oC? If the solution has a specific heat capacity of 4.18 J g-1oC-1, what is the enthalpy of solution for NaCl?

  10. First, find the heat of reaction: • qsoln = msoln ssolnDTsoln= (50.0 g + 7.85 g)(4.18 J g-1oC-1)(-2.07 oC) = -500. J qrxn = -qsoln = +500 J Now, express this per mole of NaCl: • nNaCl = (7.85 g)(1 mol / 78.45 g) = 0.100 mol DH = (+500. J) / (0.100 mol) = 5.00 x 103 J mol-1

  11. Combustion Reactions • Combustion of fuel is an exothermic process • hydrocarbons are very good at releasing energy • Involves reaction with O2 to form CO2 and H2O • For example, combustion of propane (C3H8): C3H8 (g) + 5 O2  3 CO2 + 4 H2O + heat • Note: combustion reactions are typically written for 1 mole of “fuel” • this may require fractional coefficients for O2, CO2 and/or H2O

  12. Energy Content in Fuels • In this experiment, we will see how much heat given masses of fuel produce (i.e., J per gram) • Heat flow is determined by • measuring a mass of water • recording the temperature of the water • heating the water over the burning fuel so that at least a 20 oC increase occurs • recording the new temperature • Because of Conservation of Energy, the heat lost by the fuel is gained by H2O

  13. glass rod TH2O fuel burner • set-up:

  14. Sample Calculation • To raise the temperature of 100 grams of water by 20 oC, it is necessary to burn 0.307 g of propane (C3H8). Water has s = 4.18 J g-1oC-1. What is the enthalpy of combustion of propane in kJ g-1? kJ mol-1? qH2O = -qcombustion energy conservation qH2O = mH2O sH2O DTH2O = (100 g)(4.18 J g-1oC-1)(20 oC) = 8,360 J

  15. Sample Calculation (cont.) qcombustion = -qH2O = -8,360 J Per gram: (-8360 J) / (0.307 g) = -2.72 x 104 J g-1 -27.2 kJ g-1 Per mol: (-27.2 kJ g-1)(44.09 g / mol) = -1,200 kJ mol-1

  16. Safety • Lab goggles must be worn at all times • The CaCl2 solution can be caustic • Any spilled CaCl2 must be cleaned off the balance immediately • Be careful using the fuel lampstie back long hair do not light lamps until ready

More Related